Former world number ones Roger Federer and Novak Djokovic lost in the Laver Cup doubles but Team Europe lead Team World 3-1 after day one of the event.
The pair were beaten 6-7 6-3 10-6 by American Jack Sock and South Africa's Kevin Anderson in Chicago.
Federer and Djokovic took a while to settle into the format and won the opening set but their opponents soon found their rhythm.
Earlier in the singles, Britain's Kyle Edmund defeated Sock 6-4 5-7 10-6.
There were also wins for Bulgarian Grigor Dimitrov who beat America's Frances Tiafoe 6-1 6-4 and for Belgium's David Goffin, who defeated Diego Schwartzman of Argentina 6-4 4-6 11-9.